1. Field of the Invention
This invention relates to a method of designing a fixed cutter rotary drill bit for use in the drilling of wellbores.
2. Description of the Related Art
It is known that certain parts of a fixed cutter rotary drill bit wear at a faster rate than other parts thereof. Such variations in wear have been countered, in the past, by providing the drill bit with additional cutters in the high wear areas thereof. In the past, drill bit designs were compromised because cutters having high abrasion resistance had low impact toughness and cutters having high impact toughness had low abrasion resistance. As a result of this trade-off such placement of the cutters may improve the abrasion resistance of the drill bit, but it is likely that the overall drilling efficiency is not optimized due to relatively poor impact toughness.
It is an object of the invention to provide a drill bit whereby the operating efficiency of the bit can be maintained whilst providing a bit having a desired wear profile, and to provide a method for designing such a drill bit and provide high abrasion resistance while maintaining impact toughness.
According to the present invention there is provided a method for designing a fixed cutter drill bit comprising determining a layout of cutter locations in which cutters are to be positioned upon a bit body, determining the likely wear rate for at least the cutters provided in some of the cutter locations and using the determined likely wear rates to determine whether to mount a cutter of a first type of relatively low abrasion resistance or a cutter of a second type of a relatively high abrasion resistance in each cutter location. The impact toughness of all the cutters is substantially the same.
The method of the invention is advantageous in that the positions of the cutter locations can be optimized whilst achieving an acceptable overall wear rate for the bit.
The cutters of the second type conveniently each comprise a table of polycrystalline diamond bonded to a substrate, the table of polycrystalline diamond defining a matrix of interstices containing a catalyzing material, the interstices of the matrix located within a volume close to a working surface of the cutter being substantially free of catalysing material. The volume preferably extends to a depth of at least around 0.25 mm from the working surface.
According to another aspect of the invention there is provided a method of designing a fixed cutter drill bit comprising determining a layout of cutter locations in which cutters are to be positioned upon a bit body, determining a likely wear rate for at least the cutters provided at some of the cutter locations, selecting a desired wear profile for the bit, and using the determined likely wear rates to determine the abrasion resistance of the cutter to be mounted at each cutter location to achieve the desired wear profile.
The invention also relates to a fixed cutter drill bit designed in accordance with the methods defined hereinbefore.